Despite the recognized benefits of regular physical activity for older adults, adults over the age of 65 years remain among the most inactive segments of the U.S. population. Efforts to understand the factors influencing physical activity in this important group have been limited primarily to demographic and psychosocial domains. Although the importance of the neighborhood environment in influencing a host of health, behavioral, and psychosocial outcomes has been recognized, to date, no systematic investigation of the relationship between objective and subjective environmental factors and objectively measured physical activity levels among older adults has been undertaken. The primary objective of this observational study is to investigate whether seniors living in neighborhoods conducive to walking are more physically active, after adjusting for socioeconomic status (SES), than those living in neighborhoods less conducive to walking for transportation and recreational purposes as well as other forms of physical activity. Additional questions of interest concern the moderating effects of physical function, as well as the proportion of seniors living nearby, on the environment/physical activity relationship. The proposed project takes advantage of the sampling, recruitment, and data collection methods of an ongoing NIH-funded research project aimed at integrating public health and urban planning frameworks in studying the impacts of environmental factors on physical activity levels in younger adults. Population-based sampling methods will be used to recruit adults over 65 years of age living in high walkable versus less walkable neighborhoods of varying SES levels in two regions, Seattle, WA (n = 600) and Baltimore, MD (n = 600). In addition to objectively measured physical environment (using GIS) and physical activity levels (using accelerometry), self-reported neighborhood environment, physical activity, and quality of life variables of particular relevance to older adults will be assessed twice during a 12-month period. This study, which takes advantage of the expertise of a multi-disciplinary team, will contribute important conceptual and public health information that can lead to empirically based recommendations for modifying neighborhood environment characteristics to promote physical activity and other health and quality of life outcomes among older adults.